Bone prostheses are commonly provided to patients having bone disease or injury. Such prosthetic devices are manufactured of durable materials such as metals, ceramics, or dense plastics, and are attached to the remaining bone to replace the function of defective or missing bone and joint. In a hip replacement, for example, the ball-shaped head of a femur may be replaced with a prosthetic ball attached to the proximal end of the femur by a shaft fitted within the femur shaft.
The loosening of such implants over time and with use is a significant concern. Many different techniques have been employed to try prevent such loosening. Examples include cementing the implants to the bone, the use of porous coatings on the implant to stimulate bone in-growth, and custom fit implants.
A good fit between the implant and the bone will impart a pattern of stress onto the bone which may cause it to regenerate. If the bone grows adjacent to the implant there will be less chance of loosening.
Alternatively, loss of bone around the implant may indicate poor fit between the bone and implant in certain areas and provide an early indication of future loosening or failure of the implant. For this reason, the implanted bone is often monitored after the implant is surgically implanted. Such monitoring may be performed by conventional radiographic studies, however large changes in bone density of up to 30% may be necessary before such changes become apparent on the radiograph. Preferably, digital radiographic techniques are used to provide a quantified measurement of bone density. Such techniques include dual energy x-ray absorptiometry ("DEXA") in which a measurement of bone mineral density is derived from the varying absorption of the bone of x-rays at different energies, and dual photon absorptiometry ("DPA") where a similar measurement is made using radioisotopes. Such densitometers provide quantitative in-vivo measurement of bone mineral density ("BMD"). Other digital radiographic techniques such as computed tomography ("CT") may also provide measurements of bone density, however, the metal of the prosthesis may create image artifacts in a CT image rendering the measurement of bone density in the neighborhood of the prosthesis problematic.
Conventional DEXA or DPA equipment, when used to monitor changes in bone density, may obscure subtle changes of the bone near the implant. The region of interest ("ROI") that is isolated and evaluated by such equipment may include irrelevant bone, tissue and other artifacts. Further, the ability to determine bone loss over time is limited, with such equipment, because of the difficulty in matching the data between two different measurement periods.
Recently there has been increased interest in implants constructed of composite materials, such as carbon fibers and various matrix materials, having less stiffness than the ceramic and metal materials presently used. An implant whose flexibility more closely matches that of the bone in which it is implanted is thought to eliminate "stress shielding" in which the bone around the implant is shielded from normal stresses, and thus benefit from the effects of such stress in bone remodeling and bone strengthening. A flexible implant, in contrast to stiffer implants, may pass stress through to the surrounding bone.
Many composite materials considered for implants are essentially transparent to x-rays making it difficult to accurately locate the interface between the bone and the implant for post-operative evaluation of the fit of the implant and the health of the surrounding bone.